eli5 So if the universe is constantly expanding and there is no void then what happens to light? Does it just get to the furthest expansion and wait for it to expand more? Does the expansion of the universe “stretch” light enough where it loses too much energy and stops?


Basically, where does light end up? I know that there is no void and that the universe is all that there is and it is constantly expanding, but what happens to light? Wouldn’t it reach the edge? Does cosmological redshift eventually cause the light to stop after long enough?

In: 14

The universe is expanding faster than the speed of light so it can never reach the edge. In fact the universe is expanding at an accelerated rate meaning its gaining speed. That being said there is still a lot we dont know.

Light does not have a specific “destination” in the expanding universe. It continues to travel in a straight line, but its wavelength is stretched and its frequency is lowered due to the expansion of the universe. This is known as the cosmological redshift.

As the universe expands, the space between galaxies also expands, causing the light to be stretched and redshifted. This means that the light will continue to travel, but it will become increasingly redshifted and dimmer over time. Eventually, after a very long time, the light will become so redshifted and dim that it will be difficult to detect.

However, it is important to note that the universe is not expanding into a “void” or emptiness. The universe is everything that exists, and it is expanding within itself. There is no edge or boundary to the universe, so light does not have to reach an edge or stop at any specific point. It simply continues to travel through the expanding universe.

As far as know there is no “edge” of the universe. It seems We’re either in an infinite plane or a hyper sphere. Either way light will travel forever until it hits something.

Fun fact. Everyway you point is towards where the big bang happened.

[pbs spacetime]

Universe isnt expanding at the “edges”, edges probably dont even exist, its expanding universally across its whole surface and volume. Its basically stretching.

And even the universe where our earth is, is being stretched in between you and me, but gravitational and other forces keep us together, so we dont notice it. But other celestial bodies that dont have any interaction between each other are getting further apart. Universe is being stretched everywhere, all at once.

And like someone already said here, it’s stretching faster than the speed of light, so light cant outrun it.
And we dont know but, most probably universe doesnt even have a limit, and it could even be curved in such a way that you just end up in the same place if you go far enough, like its circular in a way.

So a beam of light wouldnt reach the “edge” of the universe, it would just circle back around to where it originated from, and would keep doing that forever, until something stops it.

If you projected our four dimension space onto the 2d surface of a balloon you can see an analogue. As the balloon inflates galaxies on the surface of the balloon get further apart. Light can travel forever around the surface of the balloon and never “hit” what the balloon is “expanding into”. And from the perspective of all the 2 dimensional creatures living on the surface of the balloon their universe isn’t expanding *into* anything, they just see everything moving farther apart. In this analogy the space inside and outside of the balloon can be thought of as time. The balloon was smaller earlier, and will be bigger later. But the balloon is not expanding into physical space like you or I (or our 2 dimension friends) experience. Now convert that flat balloon surface+time existence to our three dimensions + time and you have a sense of how our universe can expand but not expand into anything. That requires an imagination better than mine.

This analogy does break down however because as far as our observations have shown the universe is most likely “flat” (or has a curvature so small our observations can’t detect it yet) and likely doesn’t curve into (like a balloon) or away from itself. As far as our experiments show, based on the error range of our observations measuring the curvature, the universe must be *at least* 23 trillion light years in diameter (we can only see 93 billion light years in diameter) or 15 million times bigger than what we can currently observe. Consider we estimate there are 2 trillion galaxies within our 93 billion light year sphere and you can imagine the massive number of galaxies that likely exist in the full universe.